The present invention relates to an endoscope used when a doctor conducts diagnosis on an body cavity and an objective lens for an endoscope arranged in a tip portion of the endoscope. In particular, the present invention relates to an objective lens for an endoscope configured such that change of an angle of view and change of curvature of field due to a manufacturing error and an assembling error are suppressed suitably, and to an endoscope on which such an objective lens is installed.
Medical endoscopes, such as a fiber scope or an electronic endoscope, are widely used by doctors for medical diagnosis for a body cavity of a patient. A tip part of an endoscope of this type is designed to be compact in size so that an insertion unit thereof can be smoothly inserted into a small space, such as a body cavity of a patient. In the tip part of the endoscope, various types of components are accommodated. It is understood that the minimum design outer dimension of the tip part of the endoscope is defined by one or more of the accommodated components (e.g., an objective lens) having a relatively large size in the tip part. Therefore, in order to downsize the tip part of the endoscope, it is desirable to downsize the objective lens by employing lenses having small sizes and by decreasing the number of lenses forming the objective lens.
In general, the objective lens is designed to have a wide angle of view to widen an observation field and thereby to enable a doctor to easily find an affected area of the patient. In particular, in a product field of digestive endoscopes adapted to diagnosis of a wide area such as a stomach, the endoscope is designed to have a considerably wide angle of view. However, in general, an optical lens shows a property that a curvature of field increases in proportion to the square of the angle of view, and thereby the image quality is deteriorated. In order to obtain a high quality image to be used for accurate diagnosis, the objective lens for an endoscope is required to be small in size and have high optical performance.
Incidentally, in principle, a manufacturing error caused during processing of optical lenses does not vary even if optical lenses vary in design size. Therefore, change of the optical performance with respect to a manufacturing error becomes more noticeable as the size of the optical lens becomes small. Similarly, change of the optical performance due to an assembling error becomes more noticeable as the size of an optical unit becomes small. The fact that the objective lens of the endoscope needs to decrease the number of lenses and thereby to increase a power to be assigned to each lens of the objective lens also badly affects the above described change of the optical performance. That is, in order to achieve the high optical performance, it is necessary to process and position each optical lens of the objective lens with a high degree of accuracy.
Regarding a small size and wide angle of view objective lens for an endoscope, change of the angle of view and change of the curvature of field due to a manufacturing error and an assembling error are seen as problems. In order to achieve the high optical performance, a manufacturer of the endoscope might considers countermeasures where strict management of manufacturing tolerances is conducted to manufacture an optical lens having an extremely low level of manufacturing error or a peripheral component arranged around the optical lens having an extremely low level of manufacturing error. However, in this case, a problem arises that the yield decreases and the manufacturing unit price increases. In addition, regarding a small size optical lens, grasping and managing the occurred error amounts are technically difficult. Therefore, strict management of manufacturing tolerances can not be employed.
Examples of concrete configurations of objective lenses of an endoscope are disclosed, for example, in Japanese Patent Provisional Publications No. HEI 2-293709A (hereafter, referred to as JP HEI 2-293709A), No. HEI 6-308381A (hereafter, referred to as JP HEI 6-308381A), No. HEI 8-122632A (hereafter, referred to as HEI 8-122632A), No. 2004-61763A (hereafter, referred to as 2004-61763A), No. 2004-354888A (hereafter, referred to as JP2004-354888A), and No. 2007-249189A (hereafter, referred to as JP2007-249189A). Regarding objective lenses disclosed in the publications, designers have tried to correct aberrations as compensation for designing an objective lens to be compact in size and to have a wide angle of view. However, according to the configurations of the objective lenses disclosed in the publications, change of the angle of view and change of the curvature of field due to a manufacturing error and an assembling error are not suppressed sufficiently, and in actuality it is difficult to achieve the suitable optical performance if strict management of manufacturing tolerances is not conducted.